Liquid crystals for electro-optical devices

This is a continuation of application Ser. No. 009,042, filed Feb. 2, 1979, now abandoned.

It is well known that nematic liquid crystals can be reoriented by the application of an electrical field. If the nematic film has been preoriented in the proper manner it can be used for making electro-optical devices (M. Tobias: International Handbook of Liquid Crystal Displays 1975-76, Ovum Ltd. London 1976; E. B. Priestley, P. J. Wojtowicz, Ping Sheng: Introduction to Liquid Crystals, Plenum Press, New York-London 1975).

In a specific application the "guest host effect" takes place in these devices due to the addition of a dichroitic dyestuff. (G. H. Heilmeier, L. A. Zanoni: Appl. Phys. Lett. 13, 91 (1968): Mol. Cryst. Liqu. Cryst. 8. 293 (1969).

However, the decay times for this kind of device under normal operating conditions are about 5 to 10 times as long as the rise times. This makes the use of these devices impractical for many applications.

The reason is that in nematic liquid crystals because of the low elastic constants, the coupling to the adjoining electrodes is too faint and the forces which cancel out the elastic deformation by the electric field are too weak.

It is therefore an object of the invention to provide for electro-optical structural elements on the basis of the principle of liquid crystals which have decay times not higher than the rise times and can therefore be switched, that is turned on or turned off more rapidly.

The invention is based on the finding that crystals of the smectic type have higher elastic constant values and therefore result in more rapid decay times. The invention is therefore based on the dichroism of smectic C-phases for obtaining color contrasts in electro-optical elements.

In particular it is based on the finding that smectic C-phases which have a positive dielectric anisotropy and which as such possess the properties of a dichroitic dyestuff can be reoriented in an electrical field and result then in a color modification of clearly discernible contrast. They are therefore suited for use in electro-optical structural elements for the purpose of modulating the transmittent or incident light and for the color reproduction of numbers, symbols and images.

The decay times of the modulating effects are considerably shorter than those in the guest-host cells made with nematic compounds and the decay times are normally shorter also than the rise times. By increasing the voltage the rise times can be further shortened so that a predetermined equalization of rise times and decay times is possible.

The structural elements made according to the invention can be used for instance in electrical clocks, pocket calculators and various measuring instruments, particularly battery operated instruments intended for indication of digital data.

For the purposes of the invention the smectic C-phases of the following type of compounds are particularly suited:

3,6-bis-(4-n-alkyl-phenyl)-1,2,4,5 tetrazine,

4-n-alkyl-4'-n-alkyloxy-3,6-diphenyl-1,2,4,5-tetrazine or

6-n-alkyl-3-(4-n-alkyloxy-phenyl)-1,2,4,5-tetrazine, the alkyl and alkyloxy chains having 1 to 12 carbon atoms

The compounds can be used by themselves or in an intermixture between different compounds of the type indicated or as a mixture with other compounds which are in the smectic C-phase state.

The smectic C-phases in these cases are obtained by cooling a preoriented nematic phase down to the smectic phase range.

The contrast effect of the devices can be improved further by adding a dyestuff exhibiting positive dichroism which is absorbing in a spectral range other than the compounds of the invention.